"The Federal Water Pollution Control Amendments of 1972" (PL92-500) and other Federal public laws and regulations reflect the national objectives and public interest in reducing the environmental pollution incident to the disposal of municipal and agricultural wastes.
An excellent review in regard to municipal wastewaters is the paper "Land Disposal--What's the Realistic View", by authors H. L. Michel, P. H. Gilbert and H. K Creed, published in Water and Wastes Engineering, June, 1974. This refers to a comprehensive study program completed for the U.S. Army Corps of Engineers by PBQ&D, Inc., Engineers, including an analysis of characteristics of wastewaters and sludges produced in the San Francisco Metropolitan Bay Area.
The opportunities and limitations regarding agricultural uses of sewage sludge are summarized in two reports published by the Council for Agricultural Science and Technology (CAST), Iowa State University, Ames, Iowa: "Utilization of Animal Manures and Sewage Sludges in Food and Fibre Production", Report 41, February, 1975, and "Application of Sewage Sludge to Croplands: Appraisal of Potential Hazards of the Heavy Metals to Plants and Animals", Report 64, November, 1976.
The potential for recovery of raw materials, now usually discarded as treated sludge, from conventional activated sludge sewage treatment facilitites is evident in the data published. Expressed in volumes per 100,000 persons, the dryweight solids content of treated sludges ranges from 8 to 17 tons per day containing about 1.15% to 6.0% of nitrogen in forms suitable for use as agricultural fertilizer. The nitrogen content is of special interest because natural gas is the principal resource used in commercial nitrogen fertilizers. For instance, about 40,000 cubic feet of natural gas are required to manufacture one ton of fertilizer nitrogen, according to Commercial Fertilizers, May, 1975, published by the Statistical reporting Service of the U.S. Department of Agriculture.
The PGQ&D, Inc., and CAST studies referred to above also provide data on the substantial amounts of heavy metals in treated sewage sludges. In terms of concentration in milligrams per kilogram of dry solids, the following concentrations are noted:
______________________________________ Element Range mg/kg = parts per million ______________________________________ Zinc 500 to 50,000 Copper 250 to 17,000 Nickel 25 to 8,000 Cadmium 5 to 2,000 ______________________________________
The potentials for resource recovery and recycling of metal values is of special interest. U.S. Geological Survey Circular 784 (1978) describes a special situation in which gold and silver are present in sewage sludge and incinerator ash in quantities providing incentives for special treatment to recover precious metals. The presence of these toxic metals is a major limitation to the use of wastewaters, treated sludge, or treated effluent for any agricultural purpose.
Land disposal of treated sewage sludge is currently practiced but is costly since the material is difficult to dewater due to its predominately cellular content. The potentials for heavy metals pollution of land and ground water require sometimes costly mitigating measures.
Uses of treated sewage sludge as agricultural fertilizer are limited to occasional applications, as repetitive use results in an accumulation of toxic heavy metals in the soil, particularly cadmium. Incineration is difficult because of water content and costly due to supplemental heat usually required to support combustion. Discharge of sludge into public waterways and the oceans is not an acceptable alternative in the opinion of many environmental experts.
According to published reports, substantial use of ensiled, composted or digested sludge from animal manure as an animal feed additive is now being practiced in the U.S.A., and the use of sewage sludge for animal feed supplementation is being tested. A report entitled, "Multielement Analysis of Animal Feed, Animal Wastes, and Sewage Sludge" by Stephan G. Capar et al. of the Division of Chemistry and Physics, U.S. Food and Drug Administration, Washington, D.C. 20204 (July, 1978), calls attention to the potentials for toxic heavy metals buildup in sludge fed to agricultural animals, particularly in multiple-pass repetitive recycling through beef cattle in close confinement feeding installations. The reference papers indicate that heavy metals are methodically concentrated in human and animal wastes at levels up to above five times the content in food and feed consumed. Biological treatment improves the feed palatability and nutrition content of such wastes; however, all such processing results in conversion of some of the organic material into gaseous form with resulting further increases in the relative content of toxic heavy metals in the remaining sludge product.
In our above referenced copending patent application, there is described an improved method for conditioning sewage wastewaters into agricultural irrigation water containing most of the influent organic materials processed into fertilizer and soil amendment materials in suspension or solution. This method is best suited for use in semiarid areas with temperate climates and long growing seasons where agricultural lands are within range of feasible distribution.
In cold weather areas where freezing conditions preclude irrigation for extended periods, and in situations where suitable agricultural lands are not situated within economic pumping distances from the urban area source of wastewater, conventional biological treatment usually is employed to produce a relatively non-polluting effluent suitable for discharge to waterways and a treated sludge residue often disposed of as waste. Usually such waste disposal involves substantial cost to reduce environmental pollution.